The present invention relates to a process for producing an alumina-silica-chromia ternary fiber.
There is known an alumina-silica-chromia based refractory fiber which is produced by melting a powder blend comprising 35-65% by weight of A1.sub.2 0.sub.3, 30-60% by weight of SiO.sub.2, 1.5-6% by weight of Cr.sub.2 O.sub.3 and small amounts of other additives or impurities in an electric furnace and thereafter fabrication of fibers by either the blowing method or the spinning method. This fiber, when compared with alumina-silica based fibers containing no chromia, is said to have less deterioration at high temperatures because of the decrease in the amount of growth of the mullite crystals above 950.degree. C. The known method of preparation of an alumina-silica-chromia fiber is to blend high-grade silica sand, alumina powder and chromia powder. However, when blended in this manner, the chromia evaporates upon melting and thus it is difficult to obtain a product of the desired composition. At the same time air pollution is caused by the chromia evaporation. The fluctuation in desired output composition becomes greater when removing the molten product as thin streams during the fiber fabrication step.
It is believed that the fluctuation in output composition is created during electrical melting. The viscosity and the electric resistance are lowered with the increase of the chromia content, hence the viscosity and the electric resistance greatly fluctuate depending on the local chromia content within the crucible. Since the melting points of the respective powder starting materials are different, i.e. 2050.degree. C. for alumina, 1723.degree. C. for silica and 2300.degree. C. for chromia, there is a time lag in melting and, as a result, a less viscous portion of the melt which has a higher chromia content is locally generated before the composition becomes uniform. Thus this portion flows out of the crucible in a considerably large amount prior to the flow of the other components. At a somewhat later time, the more highly viscous portion of the melt, having a lower content of chromia, starts to flow gradually. Since the flow output fluctuates greatly within a short time, it is very difficult to control the furnace temperatures in order to suppress such a fluctuation, and therefore the quality and composition obtained vary from product to product.
One approach to solve the above described problems has been to mix and mold all of the respective starting materials, then calcine to homogenize the
mix and finally grind the cooled homogenized composition for use as the starting material. However, this process is not economically advantageous because of the heat energy required for calcining all the starting materials beforehand and the process costs for mixing, kneading, molding, drying etc. Alternatively, it is also contemplated to previously mix and calcine chromia and silica and use the mixtures as the starting material, but experimental results reveal that as in the case of merely blending the silica sand, alumina and chromia powders, the composition of output of the molten product greatly fluctuated.
It has been discovered in the present invention that by employing a solid solution comprising 10-60% by weight of chromia in alumina as a starting material, the fluctuation in flow output can be greatly reduced as compared with the above-described processes.
Accordingly, the object of the present invention is to provide a process for producing an alumina-silica-chromia ternary fiber of uniform composition.